1. Field of the Invention
The present invention relates to a semiconductor light emitting element and, in particular, to a semiconductor light emitting element with high reliability and high brightness.
2. Description of the Related Art
Conventionally, most of light-emitting diodes (hereinafter referred to as LED) as a semiconductor light emitting element were GaP-based green. LEDs or AlGaAs-based red LEDs. In recent years, however, it becomes possible to grow a GaN-based and an AlGaInP-based high-quality crystal by MOVPE (Metalorganic Vapor Phase Epitaxy). Thus, it becomes possible to fabricate blue, green, orange, yellow, and red LEDs with high brightness.
However, the conventional AlGaInP-based LEDs have a problem in device reliability. Especially, it is an important issue to ensure reliability in relative output (i.e., emission output after a continuous current distribution test, provided that emission output before the test is 100%) and reverse voltage (Vr) of the device.
Japanese Patent No. 3195194 discloses an epitaxial wafer for a semiconductor light emitting element that comprises a first conductive-type cladding layer, an undoped active layer, a second conductive-type cladding layer and a second conductive-type current spreading layer are layered on a first conductive-type semiconductor substrate, wherein an undoped cladding layer is inserted between the second conductive-type cladding layer and the undoped active layer, as a means for increasing relative output of LED.
FIG. 1 is a cross-sectional view showing the semiconductor light emitting element disclosed in Japanese Patent No. 3195194. It is structured such that an n-type GaAs buffer layer 2, an n-type AlGaInP cladding layer 4, an AlGaInP active layer 5, a p-type AlGaInP cladding layer 6 and a p-type AlGaAs current spreading layer 8′ are sequentially layered on an n-type GaAs substrate 1, and an undoped AlGaInP layer 11 is formed between the AlGaInP active layer 5 and the p-type AlGaInP cladding layer 6. Further, a surface electrode 9 is formed on the p-type AlGaAs current spreading layer 8′, and a back surface electrode 10 is formed on the bottom of the n-type GaAs substrate 1.
From the results of its current distribution test, it is confirmed that relative output of the device can be increased due to insertion of the undoped AlGaInP layer 11 between the AlGaInP active layer 5 and the p-type AlGaInP cladding layer 6.
Although relative output of the light emitting element can be increased when the undoped AlGaInP layer 11 is inserted between the AlGaInP active layer 5 and the p-type AlGaInP cladding layer 6, reduction in reverse voltage (Vr) during the operation cannot be prevented. Namely, the semiconductor light emitting element disclosed in Japanese Patent No. 3195194 is not so reliable in reverse voltage (Vr).
In view of the above-described problem, the applicants uniquely note the diffusion of a dopant (Mg or Zn) from the second conductive-type cladding layer (the p-type AlGaInP cladding layer in Japanese Patent No. 3195194).
In the semiconductor light emitting element disclosed in Japanese Patent No. 3195194, the diffusion of the dopant into the active layer can be prevented to enhance the relative output since the undoped layer is formed between the second conductive-type cladding layer and the active layer. However, in this structure, the diffusion of the dopant into the undoped layer cannot be prevented while the diffusion of the dopant into the active layer can be prevented. Thus, when the dopant in the second conductive-type cladding layer is gradually diffused into the undoped layer, the dopant-diffused region in the undoped layer will be converted into the second conductive type. As a result, distance between the second conductive-type layer and the first conductive-type layer shortens with time. Therefore, the reverse voltage (Vr) lowers with time.